Mercury

With a diameter of 4,880 km and an orbit of only 57,910,000 kilometers, Mercury named after the Roman god of trade and travellers [1] is both the smallest and the closest to the Sun of all the planets, [2] (Ceres newly promoted to dwarf planet, Pluto newly demoted to dwarf planet and Haumea, Makemake and Eris all dwarf planets and all newly discovered are smaller). [3]

Any life on Mercury would have to be hardy indeed. In 1974 and 1975 three flybys by the Mariner 10 spacecraft (the first probe to reach Mercury) revealed a world even more inhospitable than Earth's moon, heavily cratered in some areas smooth in others a barren rock pitilessly blasted by the Sun's radiation with a mean surface temperature of 179 °C.

NASA's spacecraft Messenger (the second probe to reach Mercury and the first in orbit) performed flybys on January 14, 2008, October 6, 2008, and September 29, 2009, before final orbit insertion on March 18, 2011. The fresh, more detailed, and somewhat surprising information it's transmitting back to Earth is being added to what till now has been a rather skimpy knowledge base (water vapor in significant ammounts has been discovered in Mercury's exosphere along with signs of intense volcanic activity in its past).

BepiColombo a joint venture between ESA and JAXA is the next Mercury mission on the horizon, scheduled for launch 2014 and arrival six years later, it consists of both a planetary orbiter to map the planet and a magnetospheric orbiter to investigate its magnetosphere and magnetic field.

[1] Later to be equated with Hermes, the Greek messenger of the gods.

[2] A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round shape) and (c) has cleared the neighborhood around its orbit.

[3] A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round shape), (c) has not cleared the neighborhood around its orbit and (d) is not a satellite.

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